The response time of an oscillator is the time required by the oscillator to change oscillation frequency in response to a frequency modification command. This type of oscillator has applications in demodulators used for frequency-modulated signals, and particularly in the manufacture of SECAM chrominance signal demodulators. More generally, the oscillator has applications in a range of electronic circuits for providing a reference frequency.
FIG. 1 is a schematic representation of the operating principle of a current-controlled frequency oscillator. The oscillator includes a first current source 10 providing a current I.sub.1, and a second current source 20 providing a current I.sub.2. In the example described, currents I.sub.1 and I.sub.2 are considered to be equal, i.e., I=I.sub.1 =I.sub.2. A first capacitor 12 is connected between a first node 14 and a ground terminal 30. The first current source 10 is also connected to the first node 14. A second capacitor 22 is connected between a second node 24 and the ground terminal 30. The second current source 20 is also connected to the second node 24. The two capacitors 12, 22 have the same capacitance rating C.
A switch 32 is used to connect either the first node 14 or the second node 24 to a third current source 34 carrying a current I.sub.3. Current I.sub.3 is equal to the sum of currents I.sub.1 and I.sub.2, such that I.sub.3 =I.sub.1 +I.sub.2 =2I in the present example. A memory 36 and two comparators 16, 26 are provided to drive the switch 32 so that first and second nodes 14, 24 are alternately connected to the third current source 34.
The first comparator 16 is connected between the first node 14 and the second node 24 to measure a voltage V.sub.1 =V.sub.14 -V.sub.24, where V.sub.14 and V.sub.24 are the respective potentials of the first and second nodes 14, 24. The second comparator 26 is connected between the second node 24 and the first node 14 such that it measures a voltage V.sub.2 =V.sub.24 -V.sub.14 of the opposite polarity. The first and second comparators 16, 26 provide a pulse each time V.sub.1 or V.sub.2 reaches a threshold value V.sub.hyst that is the same for the two comparators 16, 26 in the example described. These pulses are transmitted to the memory 36 whose status changes with each pulse. The switch 32 is connected to the memory 36 and is controlled to change the switch status with each pulse.
When the oscillator is in operation, the first and second capacitors 12, 22 are alternately charged and discharged. In a first alternation, as shown in the figure, the first capacitor 14 is charged with current I.sub.1, i.e., I supplied by the first current source 10. The switch 32 connects the third current source 34 to the second node 24. The second capacitor 22 is Discharged with a current I, such that I+I.sub.2 =I.sub.3 =2I.
When the voltage V.sub.1 of the first capacitor 12 reaches value V.sub.hyst, the first comparator 16 provides a pulse that changes the status of the memory 36 and the switching status of the switch 32. The switch 32 is then switched to connect the third current source 34 to the first node 14. A second alternation then begins in which first capacitor 14 is discharged and the second capacitor 24 is charged. This second alternation is completed when the second capacitor 26 produces a pulse, i.e., when the voltage V.sub.2 of the second capacitor reaches value V.sub.hyst.
The status of memory 36, which may be considered a type of oscillator, is thus periodically modified. The frequency F of the oscillator is such that: ##EQU1## The frequency of the oscillator may be adjusted by modifying the value of current I, i.e., the intensity of the current sources. The oscillator is thus controlled by the current. The oscillation frequency can also be adjusted by modifying the value of the threshold voltage V.sub.hyst of the comparators. The oscillator is then controlled by the voltage.
As described above, the capacitors 12, 22 are respectively charged by equal currents I.sub.1, I.sub.2 and discharged by current I.sub.3. The oscillator will correctly operates provided a mean discharge current I.sub.3 is equal to the sum of the charge currents, i.e., I.sub.3 =I.sub.1 +I.sub.2. When this condition is met, the mean charge voltage of the two capacitors 12, 22 is consistent. In effect, the charge voltage of one capacitor increases when that of the other decreases. In contrast, if I.sub.3 .noteq.I.sub.1 +I.sub.2, the mean voltage of the capacitors increases or decreases and it quickly becomes impossible to use the oscillator. If I.sub.3 =I.sub.1 +I.sub.2 is never accurately checked, particularly due to the variations in the characteristics of the components, a circuit that checks and adjusts the mean voltage of the capacitors is provided. This type of control circuit is shown in FIG. 2.
FIG. 2 reproduces the components of FIG. 1 required to understand the control circuit. To provide better clarity, the comparators 16, 26 and means for driving the switch 32 are not shown. Since identical components in FIGS. 1 and 2 have the same numbers, the foregoing description applies to these components.
A terminal 40 for measuring the common mode voltage of capacitors 12, 22 is connected to the first and second nodes 14, 24 via respective resistors 42, 44. The common mode voltage V.sub.DC provides a measurement of a mean charge voltage of the capacitors 12, 22. Voltage V.sub.DC is applied to the base of a first transistor 46 connected in a voltage follower mode. The collector is connected to a power supply voltage, and the emitter is connected to a first branch of a current mirror 50.
The first branch of the current mirror is formed using a second transistor 52 having a base connected to the base of a third transistor 54. The third transistor 54 forms the second branch of the current mirror, which is connected to the switch 32 to receives current I.sub.3. The third transistor 54 in FIG. 2 operates as the third current source 34 shown in FIG. 1. Reference 56 is a DC power supply connected in series between the first and second transistors 46, 52. This power supply maintains the voltage V.sub.DC at the precise value required for correct operation of the current sources 10 and 20.
The first transistor 46 and the current mirror 50 control the discharge current I.sub.3, which maintains the mean charge voltage of the capacitors at a preset value V.sub.DC. When voltage V.sub.DC increases, the base voltages of first transistor 46 increases and the base voltages of the second transistor 52 also increases. The second transistor 52 has a collector and a base connected to the emitter of the first transistor 46 via the DC voltage source 56. The second transistor 52 therefore carries a higher current.
Since the third and second transistors form the current mirror 50, an increase in current in the first branch of the mirror, which includes the second transistor 52, causes an increase in current in the second branch of the mirror, i.e., an increase in capacitor discharge current I.sub.3. This increase in the discharge current of the capacitors tends to reduce the charge voltage at their terminals and thereby reduce voltage V.sub.DC.
Conversely, reducing the mean charge voltage of the capacitors causes a reduction in the current passing through the current mirror 50 and a reduction in the discharge current I.sub.3. The reduction in discharge current I.sub.3 causes an increase in the mean charge voltage. The discharge current control circuit thus forms a counter-reaction loop that stabilizes the voltage V.sub.DC at a balanced value. For the oscillator shown in FIG. 2 to be used in a demodulator, such as a SECAM chrominance signal demodulator, the oscillation frequency must be capable of being adjusted very rapidly to match the modulated frequency of the sub-carriers. This frequency is adjusted by instantaneous modifications to the current intensities I.sub.1 and I.sub.2 of the respective first and second current sources 10, 20.
However, the discharge current control circuit reacts slowly and the voltage of the common mode voltage V.sub.DC varies significantly before the discharge current compensates for variations in the charge currents. The oscillator is therefore unsuitable for frequency changes that are too large or too rapid. For example, a 10% variation in a frequency of 4.4 MHz in less than 1 microsecond is not possible.